1. Field of the Invention
The present invention relates in general to liquid flow devices used in apparatus for polishing semiconductor substrates, and relate in particular to a polishing solution supply system that supplies a polishing solution of a fixed concentration.
2. Description of the Related Art
In recent years, there has been a quantum leap in the density of integrated circuit devices, leading to a trend of narrower interline spacing. In, in the case of using optical photolithography, the shallow depth of focus associated with stepper optics demands extreme flatness at the focal plane.
A known method of obtaining a flat surface is to employ a polishing section (having a turntable and a polishing cloth, for example), and an object holding device for holding and pressing an object to be polished against the polishing cloth, so that polishing of the surface is achieved by sliding the object surface against the polishing cloth while supplying a polishing solution at the sliding interface. Such a polishing section produces mechanical polishing action as well as chemical polishing action by supplying an alkaline or acidic polishing solution.
In such a polishing section, variables for determining the rate of material removal from the surface (polishing rate) include not only the magnitude of the pressure against the polishing tool (e.g., polishing cloth) and the speed of relative sliding, but also the concentration of the polishing solution and the rate of supplying the solution. It is important to control the polishing rate to be constant, not only for controlling the effect of the polishing rate on the quality of the polished object but also for determining an endpoint so as to stop the polishing process properly, thereby to obtain improved productivity.
An example of such apparatus for supplying a polishing solution of a constant concentration is disclosed in Japanese Laid-Open Patent Publication, H7-52045. This apparatus comprises a stock solution tank, a first and a second solution adjusting tanks to adjust the concentration of the solution in two stages, and a circulation tank for supplying the adjusted solution to the polishing section or providing a circulating flow of the adjusted solution. Such solution storage tanks are normally high capacity tanks, ranging in capacities from 1.about.2 m.sup.3, so as to enable charging a sufficient quantity of solution to last for one day of production.
The stock solution and diluted polishing solution in this prior apparatus are not the type to be affected by exposure to air so that, although the tanks are closed with lids, effort is not made to actively exclude outside air from flowing into the tanks.
However, such a design allows escape of the interior humid atmosphere to cause drying of the interior atmosphere. Since the liquid level inside the tanks fluctuates, when the liquid around the top periphery of the tank evaporates, solute is precipitated. Repetition of such of liquid level fluctuation and solute precipitation leads ultimately to growth of the precipitate on the interior surfaces of the tanks, which then breaks off and falls into the liquid to cause blocking of liquid outlet openings, resulting not only in disruptions in production but also in fluctuation of solution concentration during the polishing operation, as well as degradation of the polished surface due to the large precipitated particles included in the polishing solution.
Therefore, a first objective of the present invention is to prevent precipitation of solutes on delivery pipes or interior walls of holding tanks so that a polishing solution of a constant solution concentration can be supplied to a polishing tool.
As described hereinbefore, to produce high quality polished products using the polishing section, it is required to supply a polishing solution of a constant concentration at a steady flow rate. Polishing solution of a given concentration is prepared by mixing a stock solution with dilution solution in a mixing tank prior to its use, and the prepared polishing solution is delivered from the mixing tank to a solution supply nozzle of the polishing section through a solution delivery pipe. It is also required that, from the viewpoints of economizing capital and operating costs, one mixing tank be used for a plurality of polishing sections. This has been achieved by providing a main distribution line where a constant flow of the polishing solution is provided and delivery pipes branching off from the main distribution line where a constant-flow pump such as a roller-operated pump is provided to pump the solution to individual polishing sections. Such arrangement can shorten the length of liquid delivery piping as well as avoid precipitation of solute from the polishing solution associated with repeated stopping and starting of individual polishing sections.
However, there are problems with this type of solution delivery arrangement. First, although a roughly constant volume of polishing solution is delivered regardless of the variations in hydrostatic pressure in the main distribution line, it is not possible to eliminate changes in the flow rate caused by deformation of flexible tubing used for the roller pumps. Second, it is inevitable that such roller pumps produce some pulsations in flow rates. Third, because of the flexibility of the tube in the roller pump, deterioration of the tube is unavoidable with age, and it is not possible to assure long-term stability for the liquid delivery system.
Further problems in such a liquid delivery system are that the quality of polished products often suffers from the lack of steady supply of polishing solution of a constant concentration to individual an polishing unit caused by system deficiencies such as differences in lengths and diameters of the delivery pipes, and fluctuations in flow rates caused by the pressure changes in the delivery circuits resulting from changing numbers of operating and non-operating polishing sections.
An second objective is, therefore, to provide an economical polishing solution supply system which is able to supply a polishing solution of a constant solution concentration at a steady rate to a plurality of polishing sections, and to provide a functionality to each polishing section so that each polishing section itself is able to control its own rate of supply of a polishing solution of a constant concentration. Such an arrangement would also allow flexibly in changing, for example, the layout of polishing sections in a plant.